Seasons is a permanent feature that updates throughout the year. In Seasons you can purchase Costumes for your Birds using Gems, and earn rewards by playing levels with Style Score. Your Style Score progress can be accessed in the second tab of the Seasons hub.
Seasons will change through the year and you will notice that different Costumes have changing Multipliers, these Multipliers are applied to your Style Score at the end of levels to help you earn Seasons rewards faster. At the start of a new Season your Style Score rewards will reset.